Field of the Invention
The invention concerns: a method to generate a spatially selective excitation in an imaging region of a magnetic resonance apparatus that precedes an acquisition of magnetic resonance data, wherein an excitation trajectory in k-space is traversed in the course of the excitation; as well as a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
Magnetic resonance imaging and its principles are widely known. A subject to be examined is introduced into an imaging region of a magnetic resonance apparatus in which a basic magnetic field with a relatively high field strength (known as the B0 field) is present. In order to be able to acquire magnetic resonance data from a subject in the imaging region (in a slice, for example), the nuclear spins of this slice are excited and the decay of this excitation is detected as a signal. Gradient fields can be generated by a gradient coil arrangement while radio-frequency excitation pulses (which are commonly designated as radio-frequency pulses) are emitted by a radio-frequency coil arrangement. A radio-frequency field (typically designated as a B1 field) is generated by the entirety of the radio-frequency pulses, and the spins of nuclei that are excited to resonance are flipped (deflected) with spatial resolution by an amount known as a flip angle, relative to the magnetic field lines of the basic magnetic field. The excited spins of the nuclei then emit radio-frequency signals that can be received and processed further by suitable reception antennas (such as the aforementioned radio-frequency coil arrangement) in order to thus acquire magnetic resonance data and reconstruct magnetic resonance images.
A multitude of possibilities have been proposed to produce excitations in a spatially selective manner. Such spatially selective (mostly multidimensional) radio-frequency pulses are often used together with fast imaging methods, for example echoplanar imaging (EPI). The resulting combination is a well-known technique in order to realize what is known as imaging with a reduced field of view (rFOV). In this way, a marked reduction of the readout and acquisition duration for the magnetic resonance data can be achieved since fewer coding steps are required. For example, a reduction of the readout and acquisition duration by a factor of 2-4 can be achieved. An additional advantage of the rFOV technique is the reduction of susceptibility-induced distortions since off-resonance effects are likewise reduced with the reduction of the total readout duration. Upon transmission, a spatially selective excitation can be used for the rFOV technique. A saturation of the nuclear spins in order to avoid aliasing artifacts is also conceivable.
However, the rFOV technique also has disadvantages that also apply to other applications of spatially selective excitations. The radio-frequency pulses of a spatially selective excitation have a markedly longer duration in comparison to the typical sinc excitations. A two-dimensional radio-frequency pulse for a spatially selective excitation thus has a duration of 20-30 ms, for example, while a sinc pulse for single slice excitation has an excitation duration of 2-3 ms. This leads to two effects. One is that more off-resonance effects that negatively affect the image quality occur during the excitation, due to the longer duration. A second effect is that the effective echo time (TE), or the contribution of the excitation to TE, is markedly lengthened. The effective echo time is defined as the difference between the point in time at which the excitation trajectory in k-space traverses the k-space center and the point in time at which the readout trajectory in k-space traverses the k-space center. However, the longer the echo time, the sooner that the signal already decays, which results in a lower SNR (signal-to-noise ratio). This is due to the tissue relaxation time (T2*, T2).
Examples of rFOV applications are found in an article by Marcus T. Alley et al., “Angiographic Imaging with 2D RF Pulses”, MRM 37:260-267 (1997), and in an article by Susanne Rieseberg et al., “Two-Dimensional Spatially-Selective RF Excitation Pulses in Echo-Planar Imaging”, MRM 47:1186-1193 (2002). In particular, it is also shown how the excitation trajectories in k-space are realized by appropriate switching (activation) of gradients. Overall, such an excitation is thus formed by at least one radio-frequency pulse and gradient pulses.
One goal of previous examinations has been to keep the resulting echo time TE as short as possible. For rFOV imaging, it has been proposed to use spatially selective, multidimensional radio-frequency pulses that are based on naturally asymmetrical excitation trajectories. One example of this is trajectories known as “spiral-in trajectories”, for example as described in the articles by Marcus T. Alley et al., “Angiographic Imaging with 2D RF Pulses”, MRM 37:260-267 (1997), and in an article by Susanne Rieseberg et al., “Two-Dimensional Spatially-Selective RF Excitation Pulses in Echo-Planar Imaging”, MRM 47:1186-1193 (2002). In such cases, in the excitation the fundamental design of the k-space center is reached only as the last point of the excitation trajectory, such that the TE contribution of the excitation is minimized. However, these approaches have other disadvantages, for example a poor robustness against hardware errors. An additional problem is that naturally asymmetrical radio-frequency pulses or complete excitations have smeared pulse responses (point spread functions—PSF). In contrast to this, spatially selective, multidimensional excitations that use a symmetrical excitation trajectory in k-space offer a more deterministic and robust point spread function, such that they are often preferred—in addition to the aforementioned articles, see also the articles by V. Andrew Stenger et al., “Three-Dimensional Tailored RF Pulses for the Reduction of Susceptibility Artifacts in T2*-Weighted Functional MRI”, MRM 44:525-531 (2000) and Johannes T. Schneider et al., “Inner-Volume Imaging In Vivo Using Three-Dimensional Parallel Spatially Selective Excitation”, MRM 2012.